The present invention relates to absorbance detectors such as those commonly used in high performance liquid chromatography (HPLC), and, more particularly, to a novel method and accompanying apparatus for automatically extending the linear dynamic range of such detectors.
The definition of absorbance is: EQU A=.epsilon.cl
where:
A=the absorbance in absorbance units (AU), PA1 .epsilon.=the molar extinction coefficient of the sample or the molar absorptivity, PA1 C=the sample concentration in moles/liter, and PA1 l=the pathlength of the measurement cell in cm. PA1 I=the transmitted light intensity PA1 I.sub.o =the incident light intensity PA1 =the transmittance
In an ideal absorbance detector the indicated absorbance is a linear function of sample concentration and Beer's Law is followed. That is: EQU A=log(I.sub.o /I) (Beer's Law) EQU and: EQU I.sub.o /I=1/T (Definition of transmittance)
where:
In an actual absorbance detector Beer's Law breaks down at higher absorbances and nonlinearities occur. There are a number of reasons for such nonlinearities aside from the chemistry of the sample. Some of the more common reasons are outlined below: